Planet Biotechnology is developing PBI-220 as a safe, effective and inexpensive therapeutic countermeasure to Bacillus anthracis - an agent threatening the U.S. civilian population. PBI-220 is expected to have broad spectrum activity against natural and engineered anthrax toxins. PBI-220, a recombinant chimeric protein, is an immunoadhesin comprised of a human receptor, a decoy against the pathogenic target, and an IgG-Fc, for a long circulating half-life and immune effector cell interaction. PBI-220 is comprised of the CMG2 extracellular domain, the human receptor for anthrax toxins, and the Fc of human IgGl. We propose to use the rabbit model of inhalation anthrax to further demonstrate the superior performance of PBI-220 derived from transgenic tobacco and then, to test, by direct comparison in a pivotal rabbit study, the performance of tobacco-derived and Chinese hamster ovary (CHO) cell-derived PBI-220. We will determine how long it is possible to delay treatment, following spore challenge, and still protect animals from death. In addition, we will directly compare the efficacy of the plant-made and CHO cell-made PBI-220s. This study will be considered a pivotal study, according to the "Animal Rule", and will be used to justify testing PBI-220, from one source or the other, in a Phase 1 clinical trial.